Tuesday, April 30, 2013

Robo Raven: A Step towards Bird-Inspired Flight

I have always been fascinated by birds. To me they represent beauty, freedom, and a design marvel. I am envious of bird watchers. What a wonderful hobby! Bird watching requires patience and traveling to exotic places where birds like to hang out. Unfortunately patience is not my forte. Also, at this stage in my life I am unable to travel to exotic places. So for now,I have compromised and have settled for the next best thing - creating and watching my own “birds”. I am interested in building robotic birds, not the kinds that look pretty on the shelf, but the ones that can actually flap their wings and fly. 
 
Eight years of experiments have taught me that designing and building robotic birds is hard, despite the apparent simplicity of the idea - flap wings to generate thrust to propel forward and use the moving air to generate lift to stay afloat. I am glad that this looked deceptively straightforward in the beginning; otherwise we would have never started on this adventurous journey. How hard can it be to build two wings and flap them using a motor? It turns out to be quite challenging if you want the bird to actually fly! This requires a long trial and error process due to the absence of accurate simulation tools. Many concepts that look good on the paper lead to a spectacular crash during the flight test, often causing a fatal injury to the robotic bird! So design iterations are painfully slow.


We had the first successful flight in 2007. +Arvind Ananthanarayanan, Wojciech Bejgerowski, +Dominik Müller  were the main architects behind this feat. Hugh Bruck, my faculty colleague at the University of Maryland, offered valuable help with the wing evaluation. We subsequently created three more flying versions using similar ideas. The last one in this series was completed in 2010. John Gerdes played a major role in building the last version in this series. Please click here to see videos of these “birds” in action. We were able to put a tiny video camera on them and get the “bird’s eye view”. We were able to launch them using a small ground robot. They were able to fly in moderate winds of around 10 mile per hour. As I mentioned before, every design flaw led to a fatal crash. But to our surprise, a very successful design also led to a fatal crash for our “birds” for a different reason. A hawk felt threatened by our "bird" and tore it apart in the mid-flight on multiple occasions!

Real birds are able to precisely control each wing during flight which enables them to do all sorts of aerobatic maneuvers. This has been a very difficult feat to achieve in bird-inspired robots. In fact, prior efforts (including our own mentioned above) utilized only simple wing motions where both wings are driven by a single motor. So motions of two wings are coupled. Minor adjustments can be made in wing motions by using small secondary actuators. But two wings cannot move completely independently. In the past, any major change in the wing motion had to be accomplished by doing a hardware change on the ground. Clearly this limited how close a robotic bird came to the real bird in terms of the flight characteristics.

I wanted to build a bird with completely independent wings that can be programmed with any arbitrary motion profiles. We did a preliminary experiment five years ago, but unfortunately it was not successful at that time. So we shelved the idea for few years. Hugh Bruck and I revived the idea again about a year ago. I am happy to report that we finally had a breakthrough last week. The students responsible for this success are Eli Barnett, John Gerdes, Johannes Kempny, Ariel Perez-Rosado, and Luke Roberts. Our new robot is based on a fundamentally new design concept. We call it Robo Raven. It features programmable wings that can be controlled independently. We can now program any desired motion patterns for the wings. This allows us to try new in-flight aerobatics that would have not been possible before. For example, we can now dive and roll. Please see below for the video of Robo Raven. 





 


The new design uses two actuators that can be synchronized electronically to achieve motion coordination between the two wings. The use of two actuators required a bigger battery and an on-board micro controller. All of this makes our robotic bird overweight. So how do we get Robo Raven to “diet” and lose weight? We used advanced manufacturing processes such as 3D printing and laser cutting to create lightweight polymer parts to reduce the weight. However, this alone was not sufficient. We needed three other tricks to get Robo Raven to fly. First, we programmed wing motion profiles that ensured that wings maintain the optimal velocity during the flap cycle to achieve the right balance between the lift and the thrust. Second, we developed a method to measure aerodynamic forces generated during the flapping cycle. This enabled us to quickly evaluate many different wing designs to select the best one. Finally, we had to perform system level optimization to make sure that all components worked well as an integrated system.

Robo Raven will enable us to explore new in-flight aerobatics. It will also allow us to more faithfully reproduce observed bird flights using robotic birds. I hope that this robotic bird will also inspire more people to choose “bird making” as their hobby!

Robotic birds (i.e., flapping wing micro air vehicles) are expected to offer advances in many different applications such as agriculture, surveillance, and environmental monitoring. Robo Raven is just the beginning. Many exciting developments lie ahead. The exotic bird that you might spot in your next trip to Hawaii might actually be a robot!